It also had fewer side effects than many standard drugs used to help those battling despair.

Researchers compared the effects of the plant hypericum perforatum - popularly known as St John's Wort - with placebos or a wide range of old and new anti-depressants, including those from the new generation of SSRI drugs, such as Prozac and Seroxat.

The findings could prompt more GPs to prescribe St John's Wort.

In Germany, it is commonly given to children and teenagers.

Experts do not know exactly how the plant lifts depression, although most believe it probably works by keeping the chemical serotonin, which is linked to positive moods, in the brain for longer.

The study's lead author, Dr Klaus Linde, from the Centre for Complementary Medicine in Munich, pooled data from 29 studies involving 5,489 patients with mild to moderately severe depression.

'Overall, the St John's Wort extracts tested in the trials were superior to placebo, similarly effective as standard anti-depressants, and had fewer side effects than standard anti-depressants,' he said.